There is no direct connection from Jerantut to Mersing. However, you can take the train to Paloh then take the taxi to Mersing. Alternatively, you can take the train to Johor Bahru Check Point, take the bus to Johor Larkin Terminal, then take the bus to Mersing.
How to get from Jerantut to Mersing
From Jerantut train to Paloh then taxi to Mersing. 7 h 49 min RM 231. From Jerantut train to Johor Bahru Check Point, bus to Johor Larkin Terminal, then bus to Mersing. 12 h 2 min RM 37. From Jerantut taxi to Mersing. 4 h 36 min RM 698. From Jerantut drive to Mersing. 4 h 36 min RM 80.